bThere are several reasons:
1) Tallaght Stadium is already declared as the venue to UEFA
2) Shamrock Rovers fans are the first prioirity and most would feel passionate about having it in Tallaght
3) The big money is in the TV rights, and the TV will pay the same money irrespective of venue
4) There is only one week to promote the game - Aviva would not automatically be a big deal financially given the probable high costs involved.
5) Shamrock Rovers can create an enormous demand relative to supply and therefore create a big "desirability factor" from within the communities they wish to target as long term supporters